1. Field of the Invention
The present invention relates to a method and an apparatus for converting color images, and, here, relates in particular to a method and an apparatus for converting input data produced by sampling a color image having pixels arranged in a first grid into output data resulting in a reproduction of the color image having pixels arranged in a second grid. In particular, the present invention relates to image scaling means for color display equipment having sub-pixels for different components.
2. Description of the Related Art
Scaling of images is employed, in particular, in displaying images on screens, the scaling being performed in a control circuit of a screen, such as a liquid crystal display screen (LCD screen). Such displays (screens) include a physically specified grid of pixels, each of the pixels including a trio of adjacent sub-pixels, each pixel including a sub-pixel for the red, green, and blue components of the image pixel, respectively, to be represented on the display. However, it is typically required for display systems including such screens to also be able to reproduce image data whose sampling grid differs from the grid which is physically implemented in the screen. If such image data is provided, the control circuit of the display has an image scaling device provided therein which is activated to produce a version of the image which has been adapted, for example by re-sampling, to the physical display grid.
Apparatuses enabling high-quality image resampling (image scaling) typically compute the resampled image data (output data) by applying multi-tap polyphase digital filters to the original image data (input data). Although it is possible to implement two-dimensional filters, it is usually more economical to implement image scaling by applying two one-dimensional filters: a horizontal scaling filter applied to each row or line in the image, and a vertical scaling filter applied to each column in the input image.
FIG. 1 shows a schematic representation of the scaling of a color image by converting the input pixels into output pixels in accordance with a conventional approach. In an exemplary manner, FIG. 1 shows six pixels 1001 to 1006 stemming from the original color image and representing the input data or input pixels. For the purposes of representing them on a screen it is now necessary to scale the input pixels so as to map same to the physically fixed grid of the display device, which is achieved by generating also six output pixels 1021 to 1026 based on the input pixels 1001 to 1006. Both for the input pixels 1001 to 1006 and for the scaled output pixels 1021 to 1066 it shall be assumed that the respective color components are evenly distributed across each pixel in the respective pixels.
By way of example, the generation of the output pixel 1024 shall now be considered. To obtain the value (e.g. intensity value) for this pixel 1024, a weighting function 104 is applied to input pixels 1001 to 1006, the weighting function 104 being related to the center of the output pixel 1024, as can be seen in FIG. 1. In the example shown in FIG. 1, the y axis is the scaling axis 106, and the distance from the center of the scaled pixel, i.e. from the center of the output pixel 1024 considered, is plotted along the x axis.
FIG. 1 shows the mathematical relationship between the input pixel data (pixels 1001 to 1006) and the scaled output pixel data produced by conventional apparatus or circuits. The luminance of an output pixel, e.g. of pixel 1024, is derived as a weighted sum of the luminances of the input pixels 1001 to 1006 centered around the position of the output pixel 1044 in the image. The precise weighting assigned to each of the input pixels is determined depending on the position of the input pixel relative to the center of the output pixel 1024, as may be seen from the curve of the weighting function 104 in FIG. 1.
An optimal choice of the weighting function 104 (filter kernel) and an optimal choice of the apparatus for efficiently realizing the weighted-sum calculations in one or more dimensions will be familiar to those skilled in the art.
In accordance with the apparatus, the conversion of the color images, represented in FIG. 1, for resampling (scaling) of same is realized by individually filtering the separated color components in the input pixel data. As is common practice, in the approach described in FIG. 1, all color components are filtered using identical sampling positions (or sampling phases), the identical sampling positions each relating to the center of the output pixel to be scaled (see pixel 1024 in FIG. 1). This approach is optimal if the images are displayed on display elements wherein the color components for each pixel are physically co-located.
However, this is relevant only in few exceptional cases, so that conventionally, no optimal resampling of the color images may be achieved, particularly in those cases where the sampling grid with which the input data or original color data has been produced differs from the physically fixed grid of the display element. In such situations, the sharpness of the image displayed is decreased, which is due to an increased passband attenuation as well as to an increase in the aliasing effect.